The present invention relates to an apparatus and a method for coding/decoding a moving image (picture) and a storage medium for storing code trains (bit stream) of the moving image. Particularly, this invention relates to such an apparatus and a method, and a storage medium for processing and storing an interlaced image signal.
In coding of a moving image, an interlaced image signal is deviated in position of scanning lines per field. Therefore, inter-image prediction and intra-image coding with respect to the interlaced image signal need be devised as compared with the case of a non-interlaced image signal.
As the procedure for coding an interlaced image signal, there are some coding systems which have already been standardized.
The J. 81 system of ITU-R is a system in which a field is used as a fundamental processing unit, and inter-image predication is adaptively switched between inter-frame and interfield prediction.
MPEG2 and DVC are another system in which a frame is used as a fundamental processing unit, and processing per field and frame unit are switched in finer macro block unit.
An example of a coding apparatus and decoding apparatus for the interlaced image described above is shown in FIG. 1.
In FIG. 1, an interlaced image signal input via input terminal 1 is coded and compressed by an interlaced image coder 71 into a code train. The code train (bit stream) is supplied to an interlaced image decoder 72.
The interlaced image decoder 72 is paired with the interlaced image coder 71, to reproduce the interlaced image signal that is output via output terminal 25.
Further, there has been contemplated, as disclosed in Japanese Patent Laid-Open No. 3(1991)-132278 (Japanese Patent Application No. 1(1989)-271006 entitled "VIDEO SIGNAL TRANSFORM APPARATUS", a method for converting an interlaced image signal of 60 fields per second into an interlaced image signal of 30 frames per second to provide non-interlaced image coding.
However, in this case, a violent flicker occurs when 30 frames per second are displayed on a display apparatus as they are. Therefore, one frame is divided into two fields to obtain 60 fields per second before display. The coding system used here is a non-interlaced coding system like MPEG1.
In the case of an image of low resolution in a system, such as, MPEG1, only one of two fields of an interlaced image signal is used while the other is cancelled, to obtain a non-interlaced image signal of 30 frames per second to be coded.
Further, as an image format, there has been known a progressive image of 60 frames per second. This is called a sequential scanning because scanning lines are present also on scanning line portions decimated in an interlaced image. The fundamental scanning line construction of the progressive image is the same as that of the non-interlaced image, which can be regarded as a high frame rate non-interlaced signal.
The progressive image signal can be displayed on a display apparatus without flicker but a horizontal deflection frequency or a video signal band is doubled. Thus, the progressive image signal cannot be displayed on a usual television. As the coding system, a non-interlaced coding system like MPEG1 can be employed. However, double processing speed is necessary because the progressive image has 60 frames per second.
The coding efficiency in the case where an image of each format is coded, that is, a necessary information amount (bit) per pixel will suffice to be the least in the progressive image of 60 frames per second, next in the non-interlaced image of 30 frames per second, and the interlaced image requires the largest number of codes.
The coding efficiency of the interlaced image is poor due to the presence of an aliasing component included in each field image. As viewed from the field, the interlaced image is not applied with vertical filtering fulfilled with a sampling theorem and includes many aliasing components.
In the case where an image is moving, the interlaced image can be processed by motion compensation in interfield prediction. However, the folded distortion tends to change per field so that the prediction is not precise to lower the coding efficiency. The coding efficiency is further lowered due to high frequency components increase in the intra-picture processing.
On the other hand, in the case of the progressive moving image of 60 frames per second and the non-interlaced moving image of 30 frames per second, the difference therebetween lies only in the frame rate. Either of the image of higher frame rate involves a closer distance between frames in time with less image change between frames. This results in precise inter-image prediction to enhance the coding efficiency. With respect to the intraframe coding, no difference is present between the progressive and the non-interlaced images.
Further, in case of interlaced image signal coding, the coding efficiency is not enhanced in the processing per field. Because the field image or the predictive residue includes many aliasing components. Even in the frame/field adaptive prediction, quick images are subjected to the field processing and the situation is similar to that described above.
As described above, the interlaced signal is inferior in the coding efficiency to the non-interlaced signal. Further, when the interlaced signal of 60 fields per second is converted into the non-interlaced signal of 30 frames per second, the movement of a reproduced image is not smooth as compared with an original interlaced image.